How to have off grid inverter supply power only if able to?

[2strokeforever]

I just reread the whole thread and I must admit I missed (well not rly understood) what @2strokeforever suggested.

If I understand this correctly you would make the junction in charging batteries, 2 MPPTs that talk to each other, one of them primary charger (solar), the other one secondary (grid). Then inverter from batteries to AC that powers the load. Only source here would be inverter, but batteries would have 2 sources, not the ATS. That is interesting idea. Only downside is that going always via batteries means relatively significant loss on {AC->Battery->Inverter->Load} double conversion. That woud prolong ROI, it is optimized for availability (I have separate UPS for that) and not cost saving.
Fresh approach nonetheless.

There is no way to do what you want and save money.
If you were grid tied and got credits you could save money, but thats the only way.

 

[hidegety]

Everytime you switch, there is a glitch in the output wave form. Not good for sensitive devices.

Switches have a limited number of times they are designed to switch during their operating life.

I see. I will check the device specification, tx for tip. As for the glitch -> is there some consensus on maximum switch time (e.g. is 8ms too long)? I guess it depends on type of load. I would also say all computer equipment power sources have some limited ability to handle small glitches in waveform - especially considering they all convert everything down to 12 or 5V - they are full of caps, inductors, diodes and trafos. None of that equipment runs on 230V.
Also -> what would you say should be max switch cycles so that it doesn't become excessive - in scope of solar setup?

 

[TomC4306]

like to power our server rack from solar as much as possible, optimized for money saving.

we just pay rent (of which power costs are part)

If you go through all this, and you use less electricity, will your rent go down to reflect that?
Or would you just be improving your landlords profit margin?

 

[DIYrich]

I see. I will check the device specification, tx for tip. As for the glitch -> is there some consensus on maximum switch time (e.g. is 8ms too long)? I guess it depends on type of load. I would also say all computer equipment power sources have some limited ability to handle small glitches in waveform - especially considering they all convert everything down to 12 or 5V - they are full of caps, inductors, diodes and trafos. None of that equipment runs on 230V.
Also -> what would you say should be max switch cycles so that it doesn't become excessive - in scope of solar setup?

Transfer time for UPS is up to 12ms. There are 120 peaks per second in a 60hz wave form for a rectifier to convert the AC to DC. Anything greater than 4ms risks missing one of the peaks. I have no idea how many peaks it can miss before causing a problem. What is the transfer time of your ATS?

In any case, a few transfers per day is not an issue. The problem is when your system is hovering around transfer state. It could cycle every few seconds. That is hard on your switch and equipment.

 

[hidegety]

If you go through all this, and you use less electricity, will your rent go down to reflect that?
Or would you just be improving your landlords profit margin?

Good question. I pay for power separately. Total property consumption (it is a large commerce/light industry area) costs splitted proportionally according to our portion of consumption.
So no, every KWH we do not take from them is KWH we will not have to pay.

 

[hidegety]

Transfer time for UPS is up to 12ms. There are 120 peaks per second in a 60hz wave form for a rectifier to convert the AC to DC. Anything greater than 4ms risks missing one of the peaks. I have no idea how many peaks it can miss before causing a problem. What is the transfer time of your ATS?

According to documentation 8-12ms typical, with 18 max guaranteed (for 50Hz).

In any case, a few transfers per day is not an issue. The problem is when your system is hovering around transfer state. It could cycle every few seconds. That is hard on your switch and equipment.

True, the switching must be done above the boundary, with enough reserve so that it doesnt come to that. All question of properly seting switch limit voltages.

 

[DIYrich]

All question of properly seting switch limit vovoltages.

I don't think that is easy to do without a battery.

 

[hidegety]

I don't think that is easy to do without a battery.

Oh, I see. Battery would drop the solar voltage down to its voltage, as opposed to solar panels solo - where it is more current that fluctuates (only losing voltage if heavily shaded).

So, If the system is [panels / battery / inverter+mmpt] then voltage is a good indicator of the off grid system ability to fully power the load.
Without batteries - can I set inverter so, that it powers the output only in case, it is able to supply some min load (min watts)?

And extension of that question: can I somehow signalize inverter what is ATM current expected load (load it must support).

Which brings us neatly to the beginning of the thread: How does off grid inverter know if it is capable to support the load (that for sake of the argument is currently powered by diesel generator)? Or am I beating a dead horse here?

 

[DIYrich]

Oh, I see. Battery would drop the solar voltage down to its voltage, as opposed to solar panels solo - where it is more current that fluctuates (only losing voltage if heavily shaded).

Charge controller or Inverter drops the Solar voltage to Battery voltage.

So, If the system is [panels / battery / inverter+mmpt] then voltage is a good indicator of the off grid system ability to fully power the load.
Without batteries - can I set inverter so, that it powers the output only in case, it is able to supply some min load (min watts)?

Battery voltage is a good indicator of whether there is enough battery power to let the inverter provide maximum output. When Solar charges up to that minimum level, then inverter turns on. It will continue to run as long as there is enough PV to keep the batteries charged, or until the batteries drain down to the cutoff point.

And extension of that question: can I somehow signalize inverter what is ATM current expected load (load it must support).

Only if the inverter has the ability to connect to CT's.

Which brings us neatly to the beginning of the thread: How does off grid inverter know if it is capable to support the load (that for sake of the argument is currently powered by diesel generator)? Or am I beating a dead horse here?

You size the inverter for your needs. It is your responsibility to ensure that you do not overload the inverter. There are Load Management devices that can turn on/off individual circuits to avoid overloading the inverter.

 

[hidegety]

Thank you all very much for your patience and suggestions. I think I now have clearer understanding of what needs to be done in order to bring this setup to fruition.
The realization will take me couple of months, I will post some results and experiences once it is done - just in case someone else comes across this issue.